Welding method

ABSTRACT

A welding method for improving the durability and strength of fusion weld joints in metal structures; the method is especially beneficial for metal structures fabricated from nickel-titanium alloy (nitinol) and for medical devices.

RELATED APPLICATIONS

The present application is a divisional of application Ser. No.08/735,031, filed Oct. 22, 1996, now U.S. Pat. No. 6,000,601 which isincorporated herein by reference in its entirety.

BACKGROUND OF THE INVENTION

Fusion welding techniques, such as laser welding, electron beam welding,tungsten inert gas welding, plasma welding and others, commonly leave apool of molten metal at the end of the weld pass that solidifies afterheat input to the weld has terminated. The terminal pool is commonly theweakest portion of the weld and is often the site of fracture initiationunder moderate to low stress. Welds produced in nickel-titanium alloys(nitinol) are particularly prone to this problem.

SUMMARY OF THE INVENTION

The invention comprises a method of moving the location of the terminalpool in fusion welds to better accommodate high stress areas. This isaccomplished by utilizing a multiple pass weld technique. The weldingtechnique utilizes weld passes which initiate at remote portions of thearea to be welded such as each end of a joint to be welded, andterminate within the interior of the weld area at or near a commonmeeting point. The multiple passes may overlap within the weld butoverlap is not required. This technique is especially useful in deviceshaving portions comprised of nitinol which require welding, such as inmedical stents.

BRIEF DESCRIPTION OF THE FIGURES

Reference is made to the accompanying drawings in which illustrativeembodiments of the invention are shown and from which the novel featuresand advantages of the invention will be apparent.

FIG. 1 is a schematic showing of a multiple pass method of weldingaccording to the invention wherein the two passes meet at a commonpoint.

FIG. 2 is similar to FIG. 1 showing overlap between the multiple passes.

FIG. 3 is a perspective view of one form of stent which may make use ofthe invention.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS OF THE INVENTION

Referring to FIGS. 1 and 2, it will be seen that a metal workpiece to bejoined consisting of two elongate pieces 10 and 12 respectively broughtinto contact at 14 may be fusion welded according to the invention bytwo passes. Pass one starts as indicated (START PASS 1) at an outerportion 16 of a weld area and moves to the right in the Figure to end ina terminal pool as indicated at END PASS 1. A second pass (START PASS 2)begins at 18 and moves to the left in the Figure ending as indicated ina terminal pool at END PASS 2. In this embodiment the welding methodutilizes two weld passes which initiate at each end of the joint andterminate within the interior of the weld joint the progression of theweld passes is shown schematically by the succession of circular areas.

FIG. 2 similarly uses two passes which move toward each other. However,they overlap as shown in the Figure where it can be seen that PASS 1beginning at 16 and moving to the left in the Figure and PASS 2beginning at 18 and moving to the right in the Figure can be seen tooverlap within the weld to form a terminal pool.

For the purposes of this invention, as already indicated above, any ofthe various known forms of fusion welding may be utilized. Laser weldingis presently most preferred. The passes may be sequential orsimultaneous.

Referring to FIG. 3 it will be seen that an illustrative medical stentmay include a skeletal frame generally indicated at 2, preferably formedfrom a single nitinol wire 4, bent to form a desired configuration. Thewire 4 includes a plurality of abutting straight portions 6 which arejoined to each other by welding utilizing the method described in FIGS.1 or 2 above. As already pointed out, a nitinol stent especiallybenefits from this invention because nitinol stents are particularlyprone to the problem of fracture initiation at the weld ends whereterminal pools have been formed utilizing prior art welding techniques.The stent shown is fully disclosed in U.S. Pat. Nos. 5,354,308 and5,395,390 and the entire content of both of these patents areincorporated herein by reference. The stent shown in FIG. 3 is shownhere as an exemplary stent which may make use of the method of theinvention although the method will find application in other stentconfigurations and in other metallic devices which require welding.

This disclosure is intended to be illustrative and not exhaustive. Itwill suggest many variations and alternatives to one of ordinary skillin this art. All these alternatives and variations are intended to beincluded within the scope of the attached claims. Those familiar withthe art may recognize other equivalents to the specific embodimentsdescribed herein which equivalents are also intended to be encompassedby the claims attached hereto.

What is claimed is:
 1. A medical device, the medical device beingimplantable within the human body and being formed of a fusion weldablematerial shaped in a substantially tubular configuration, the medicaldevice having bent sections axially adjacent to one another, said bentsections being connected by fusion welding forming a weld line, the weldline having a first initiation point and a second initiation point and asubstantially centrally located region between the first initiationpoint and the second initiation point along the weld line, wherein theweld line extends substantially the length of the bent sections, thecentrally located region being in a relatively lower stress region ofthe weld line as compared to the first and second initiation points, theweld line further comprising a terminal pool located in the centrallylocated region.
 2. The medical device of claim 1, wherein the fusionweldable material comprises nitinol.
 3. The medical device of claim 1,wherein the medical device is a wire metal stent, the stent beinggenerally tubular and being expandable from a contracted stated to anexpanded state.
 4. The medical device of claim 3, wherein the terminalpool is located at point on the weld line equidistant from theinitiation points.
 5. The medical device of claim 1, formed by themethod shaping and bending the fusion weldable material forming thesubstantially tubular configuration having the axially adjacent bentsections, coupling the adjacent bent section to form a work piece areas,the method further involving multiple passes comprising the steps ofmaking a first fusion welding pass over a portion of a work piece areaof the metal implantable medical device to be welded and making a secondfusion welding pass over another portion of the work piece to be welded,thereby connecting the bent sections and forming the weld line, the twopasses being directed toward each other and toward a common interiorpoint whereby the terminal pool is formed in a low stress ornon-critical area of the work piece area.
 6. The medical device of claim5, wherein the work piece area is a nitinol weld area.
 7. The medicaldevice of claim 5, wherein the medical device is a wire metal stent, thestent being generally tubular and being expandable from a contractedstated to an expanded state.
 8. The medical device of claim 5 whereinthe extent of the two passes mutually overlap at a common interiormeeting point.
 9. The medical device of claim 5 wherein the extent ofthe two passes ends substantially at a common interior meeting point.10. The medical device of claim 5 wherein the two passes occursequentially.
 11. The medical device of claim 5 wherein the two passesoccur simultaneously.
 12. The medical device of claim 5, wherein theterminal pool is located at point on the weld line equidistant from theinitiation points.